Mixtures of physical absorption solvents and ionic liquids for gas separation

ABSTRACT

The invention comprises an absorbent composition and process for purification of gaseous mixtures. The composition comprises a mixture of a physical absorption solvent and an ionic liquid. It was found that the mixtures provided improved absorption of a gas component, such as carbon dioxide, when compared physical absorption solvents.

BACKGROUND OF THE INVENTION

The separation of carbon dioxide from gas mixtures, such as natural gas,flue gas, syngas and shale gas, is of industrial importance. The removalof carbon dioxide is necessary to improve the fuel quality of thenatural gas or to use syngas. In addition, the combination of carbondioxide and water can be corrosive to metal pipes, which makes theremoval of CO₂ necessary for transportation of natural gas. Also, carbondioxide is a greenhouse gas that needs to be captured from flue gases toavoid harming the environment.

Current removal technologies, such as physical absorption solventsmethanol, N-methylpyrrolidone, dimethyl ether of polyethylene glycol,and propylene carbonate, have many drawbacks. Common issues with thesesolvent removal systems are low operating temperatures and highoperating pressures. In addition, effluent washes may be needed forsolvents lost in the stream. Physical absorption solvents tend to befavored over chemical solvents when the concentration of acid gases orother impurities is very high. Unlike chemical solvents, physicalabsorption solvents are non-corrosive, requiring only carbon steelconstruction.

In recent years, it has been found that some ionic liquids are useful inthe capture of carbon dioxide. For example, in WO 201217183, a processwas disclosed for separating carbon dioxide from a gaseous streamthrough chemisorption by 1-ethyl-3-methylimidazolium (emim) or1-propyl-3-methylimidazolium (pmim) containing ionic liquids with acarboxylate salt and the presence of guanidinium acetate or1-butyl-3-methylimidazolium (bmim) acetate.

In addition, prior art has described the use of amine solutions combinedwith ionic liquids for removal of carbon dioxide and other impurities(US 2012/0063978 A1).

There are approximately four types of physical absorption solvents thatare used for the purification of gas mixtures. These include dimethylether of polyethylene (DEPG), methanol, N-methyl-pyrrolidone (NMP) andpropylene carbonate (PC).

Dimethyl ether of polyethylene glycol (DEPG) is a mixture of dimethylethers of polyethylene glycol (CH₃O(C₂H₄O)_(n)CH₃ (where n is between 2and 9) used to physically absorb H₂S, CO₂, and mercaptans from gasstreams. Solvents containing DEPG are licensed, manufactured, or used inprocesses by several companies including Coastal Chemical Company (asCoastal AGR), Dow (Selexol), and UOP (Selexol process). Other processsuppliers such as Clariant GmbH of Germany offer similar solvents.Clariant solvents are a family of dialkyl ethers of polyethylene glycolunder the Genosorb® name. DEPG can be used for selective H₂S removalwhich requires stripping, vacuum stripping, or a reboiler. The processcan be configured to yield both a rich H₂S feed to the Claus unit aswell as bulk CO₂ removal. Selective H₂S removal with deep CO₂ removalusually requires a two-stage process with two absorption andregeneration columns. H₂S is selectively removed in the first column bya lean solvent that has been thoroughly stripped with steam, while CO₂is removed in the second absorber. The second stage solvent can beregenerated with air or nitrogen for deep CO₂ removal, or using a seriesof flashes if bulk CO₂ removal is required. DEPG also dehydrates the gasand removes HCN. Compared to the other solvents, DEPG has a higherviscosity which reduces mass transfer rates and tray efficiencies andincreases packing or tray requirements, especially at reducedtemperatures.

There are a number of methanol processes for acid gas removal includingthe Rectisol process (licensed by Lurgi AG) and Ifpexol® (Prosernat).The Rectisol process was the earliest commercial process based on anorganic physical absorption solvent and is widely used for synthesis gasapplications. The process operates at a very low temperature and iscomplex compared to other physical absorption solvent processes. Themain application for the Rectisol process is purification of synthesisgases derived from the gasification of heavy oil and coal rather thannatural gas treating applications. The two-stage Ifpexol process can beused for natural gas applications. Ifpex-1 removes condensablehydrocarbons and water, and Ifpex-2 removes acid gas. Methanol has arelatively high vapor pressure at normal process conditions, so deeprefrigeration or special recovery methods are required to prevent highsolvent losses. Water washing of 3 effluent streams is often used torecover the methanol. The Rectisol process typically operates below 32°F. (0° C.) and may be operated at temperatures as low as −95° F. (−70.5°C.). The process usually operates between −40° F. and −80° F. (−40° C.and −62° C.).

Solubilities of H₂S and COS in methanol are higher than in DEPG.Rectisol's complex flow scheme and the need to refrigerate the solventcan be disadvantages with respect to higher capital and operating costs.The supply of refrigeration at low temperatures requires much power.However, this disadvantage can be outweighed by a considerable reductionof the solvent flow rate for CO₂ removal as compared to other physicalabsorption solvent processes. Acid gas solubility in physical absorptionsolvents increases significantly as the temperature decreases. Lowtemperature also reduces solvent losses by lowering the vapor pressureof the methanol in the product streams. If H₂S is to be removed from agas with CO₂ remaining in the treated gas, DEPG and NMP are moresuitable than methanol.

The Purisol Process which uses NMP (N-methyl-2-pyrrolidone) is licensedby Lurgi AG. The flow schemes used for this solvent are similar to thoseused for DEPG. The process can be operated either at ambient temperatureor with refrigeration down to about 5° F. (−15° C.). NMP has arelatively high vapor pressure compared to DEPG or PC, and the licensorrecommends water washing of both the treated gas and the rejected acidgases for solvent recovery. Obviously, NMP cannot be used forsimultaneous gas dehydration if a water wash is used. In general, NMPrecovery with water is not necessary if the Purisol process is operatedat subambient temperatures. NMP has been reported to have the highestselectivity of all the physical absorption solvents considered here forH₂S over CO₂. COS is not as soluble as H₂S, but it is hydrolyzed by theNMP solvent. The Purisol process is particularly well suited to thepurification of high-pressure, high CO₂ synthesis gas for gas turbineintegrated gasification combined cycle (IGCC) systems because of thehigh selectivity for H₂S.

The Fluor Solvent process which uses propylene carbonate (PC) islicensed by Fluor Daniel, Inc. and has been in use since the late1950's. PC is available as JEFFSOL® PC solvent and is particularlyadvantageous in treating syngas. PC has an advantage over the othersolvents when little or no H₂S is present and CO₂ removal is important.PC has lower solubilities of the gas being purified: light hydrocarbonsin natural gas and hydrogen in synthesis gas. This lower solubilityresults in lower recycle gas compression requirements for the gasflashed from the rich solvent at intermediate pressures, and lowerhydrocarbon losses in the CO₂ vent gas stream.

Ionic liquids are capable of solubilizing or reacting with polarmolecules. Ionic liquids are comprised of a cation and anion and areliquid at or below the process temperature. Ionic liquidscharacteristically are non-flammable, non-degradable, viscous, thermallystable and have a low vapor pressure. Many of these characteristicswould be solutions to the problems of current carbon dioxide removaltechnology. While many of the characteristics of ionic liquids arebeneficial, the high viscosity of ionic liquids may be challenging. Ithas now been found that ionic liquids can be added to physicalabsorption solvents in a variety of weight percents to alleviate theviscosity issue, and improve the performance of the solvent.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE shows the isotherms using a physical absorption solvent as acontrol and mixtures of physical absorption solvents with ionic liquids.

DESCRIPTION OF THE INVENTION

One embodiment of the invention involves a composition comprising anionic liquid and a physical absorption solvent. The physical absorptionsolvents that may be used include, but are not limited to, dimethylethers of propylene glycol, N-methyl-2-pyrrolidone, methanol, propylenecarbonate, poly(propylene glycol) di-methyl ether (PPGDME),poly(propylene glycol) di-acetate (PPGDAc), poly(butylene glycol)di-acetate (PBGDAc) with linear or branched C4 monomers, poly(dimethylsiloxane) (PDMS), perfluoropolyether (PFPE), glycerol tri-acetate (GTA),acetone, methyl acetate, 1,4-dioxane, 2-methoxyethyl acetate,2-nitropropane, n,n-dimethylacetamide, acetylacetone, 1-nitropropane,isooctane, 2-(2-butoxyethoxy)ethyl acetate, n-formylmorpholine,2-butoxyethyl acetate, and n-tert-butylformamide. Preferably, thephysical absorption solvent is a dimethyl ether of propylene glycol,N-methyl-2-pyrrolidone, methanol and propylene carbonate. The cation ofthe ionic liquids may be selected from, but is not limited to, thefollowing: ammonium, phosphonium, imidazolium, pyrazolium, pyridinium,pyrrolidinium, sulfonium, piperidinium, caprolactamium, guanidinium andmorpholium. The anion of the ionic liquid may be selected from, but isnot limited to, the following: halides, carboxylates, sulfonates,sulfates, tosylates, carbonates, phosphates, phosphinates, borates,cyanates, bis(trifluoromethylsulfonyl)imides, and aprotic heterocyclicanions. The ionic liquid is preferably selected from the groupconsisting of phosphonium and imidazolium acetate ionic liquids. Thecomposition may further comprise water.

The composition may comprise about 1-99 vol % ionic liquid and about1-99 vol % absorption solvent. It may comprise about 5-95 vol % ionicliquid and about 5-95 vol % physical absorption solvent. In otherembodiments, the composition comprises about 25-75 vol % of the ionicliquid and about 25-75 vol % of the physical absorption solvent. Inanother embodiment of the invention, the composition comprises about40-60 vol % of the ionic liquid and about 40-60 vol % of the physicalabsorption solvent.

The invention also comprises the method of purifying gaseous mixtures byuse of these compositions. This method comprises contacting a gasmixture with a mixture of an ionic liquid and a physical absorptionsolvent in an absorbent zone wherein the ionic liquid and physicalabsorption solvent mixture absorbs at least one component from said gasmixture, and then the ionic liquid and physical absorption solventmixture is regenerated to remove the absorbed component or components.The method is useful for carbon dioxide containing gas mixtures. Amongthe gas mixtures that may be treated are natural gas, flue gas, syngas,and shale gas.

In the method, the physical absorption solvents that may be usedinclude, but are not limited to, dimethyl ethers of propylene glycol,N-methyl-2-pyrrolidone, methanol, propylene carbonate, poly(propyleneglycol) di-methyl ether (PPGDME), poly(propylene glycol) di-acetate(PPGDAc), poly(butylene glycol) di-acetate (PBGDAc) with linear orbranched C4 monomers, poly(dimethyl siloxane) (PDMS), perfluoropolyether(PFPE), glycerol tri-acetate (GTA), acetone, methyl acetate,1,4-dioxane, 2-methoxyethyl acetate, 2-nitropropane,n,n-dimethylacetamide, acetylacetone, 1-nitropropane, isooctane,2-(2-butoxyethoxy)ethyl acetate, n-formylmorpholine, 2-butoxyethylacetate, and n-tert-butylformamide. Preferably, the physical absorptionsolvent is dimethyl ether of propylene glycol, N-methyl-2-pyrrolidone,methanol and propylene carbonate. The cation of the ionic liquids may beselected from, but is not limited to, the following: ammonium,phosphonium, imidazolium, pyrazolium, pyridinium, pyrrolidinium,sulfonium, piperidinium, caprolactamium, guanidinium and morpholium. Theanion of the ionic liquid may be selected from, but is not limited to,the following: halides, carboxylates, sulfonates, sulfates, tosylates,carbonates, phosphates, phosphinates, borates, cyanates,bis(trifluoromethylsulfonyl)imides, and aprotic heterocyclic anions. Thepreferred ionic liquids may be selected from the group consisting ofphosphonium and imidazolium acetate ionic liquids. The composition mayfurther comprise water. The mixture of physical absorption solvent andionic liquid may comprise from about 5-95 vol % ionic liquid and fromabout 5-95 vol % physical absorption solvent. In another embodiment, themixture comprises from about 25-75 vol % of the ionic liquid and fromabout 25-75 vol % of the physical absorption solvent. The mixture maycomprise from about 40-60 vol % of said ionic liquid and from about40-60 vol % of said physical absorption solvent. The method isparticularly useful for gas mixtures containing carbon dioxide. Amongthe gas mixtures that may be treated are natural gas, flue gas, syngas,and shale gas.

The addition of an ionic liquid to a physical absorption solvent has thecapability to eliminate the need for refrigeration and effluent washing.The addition of ionic liquids to physical absorption solvents at a rangeof concentrations demonstrates an increase in performance compared tothe physical absorption solvents. Among the benefits of ionic liquidaddition to physical absorption solvent are an increased performance incapacity and a lower possible operating pressure.

In the present invention, a phosphonium or imidazolium based ionicliquid is added to a physical absorption solvent. In an autoclave, theionic liquid and physical absorption solvent mixture was exposed to acarbon dioxide and methane gas mixture, 2068 kPa (300 psi) of 10 mol %CO₂/CH₄. The mixture is stirred for 1 hour at room temperature, and thena sample from the gas headspace is taken and analyzed.

The DEPG and NMP ionic liquid mixtures performed better than methanolionic liquid mixtures at each solvent wt % for the imidazolium ionicliquids. Methanol and ionic liquid mixtures performed better than NMP orDEPG ionic liquid mixtures at each solvent wt % for the phosphoniumionic liquids. Adding a small amount of ionic liquid unexpectedlyincreased the performance of the absorption solvent. These results leadto a conclusion that combination of physical absorption technology withionic liquids increased the performance of the solvent used in thephysical absorption.

Laboratory Procedure

The ionic liquid and physical absorption solvent were combined andstirred until well mixed in a glass insert with a magnetic stir bar. Theglass insert is then placed in a 75 mL Parr reactor. The reactor isflushed with nitrogen and pressurized with a carbon dioxide/methane gasmixture. After stirring for 1 hour, a gas sample of the headspace isremoved for GC analysis.

Example with CO₂/CH₄

Tris(propyl/butyl)methylphosphonium acetate (2.20 g, 0.0087 mol) wasadded to DEPG (2.13 g, 0.0085 mol) in a glass insert for a 75 mLautoclave and stirred until well mixed with a magnetic stir bar. The 75mL autoclave was loaded with the glass insert containing the ionicliquid mixture. The autoclave was flushed with nitrogen and thenpressurized with 2068 kPa (300 psi) of a 10 mol % carbon dioxide/methanegas mixture. The mixture was stirred for an hour with a magnetic stirbar at 500 rpm. A sample of the headspace was obtained and submitted forGC analysis. Results indicated 22% of the carbon dioxide had beenremoved.

Example of Isotherm Experiment

Tris(propyl/butyl)methylphosphonium acetate (2.92 g, 0.0100 mol) wasadded to DEPG (0.55 g, 0.0173 mol) in a glass insert for a 75 mLautoclave and stirred until well mixed with a magnetic stir bar. The 75mL autoclave was loaded with the glass insert containing the ionicliquid mixture. The autoclave was flushed with nitrogen and thenpressurized with the desired amount of carbon dioxide. The mixture wasstirred 500 rpm until an equilibrium pressure was obtained. The observeddecrease in pressure was attributed to the absorption of carbon dioxide.

Results

TABLE 1 Capacity of a Phosphonium Based Ionic Liquid and PhysicalAbsorption Solvents capacity Absorption Solvent (mol CO₂ removed/mol IL)NMP 0.022 DEPG 0.025 MeOH 0.007 PmixOAc + NMP (14 wt %) 0.094 PmixOAc +MeOH (14 wt %) 0.280 PmixOAc + DEPG (13 wt %) 0.142 *2068 kPa (300 psi)of (10 mol %) CO₂/CH₄ added NMP = N-methylpyrrolidone DEPG =dimethylether of polyethylene glycol MeOH = methanol

TABLE 2 Capacity of an Imidazolium Based Ionic Liquid and PhysicalAbsorption Solvents capacity Absorption Solvent (mol CO₂ removed/mol IL)NMP 0.022 DEPG 0.025 MeOH 0.007 bmimOAc + NMP (16 wt %) 0.289 bmimOAc +MeOH (12 wt %) 0.235 bmimOAc + DEPG(17 wt %) 0.289 *2068 kPa (300 psi)of (10 mol %) CO₂/CH₄ added NMP = N-methylpyrrolidone DEPG =dimethylether of polyethylene glycol MeOH = methanol

In particular, it was found that ionic liquids increased the capacity ofphysical absorption solvents at low pressures of carbon dioxide. Thetype of solvent plays a role in the performance of the ionic liquid.

The invention claimed is:
 1. A composition comprising an ionic liquidand a physical absorption solvent wherein said ionic liquid comprises acation selected from the group consisting of ammonium, phosphonium,pyrazolium, pyrrolidinium, sulfonium, piperidinium, caprolactamium,guanidinium, and morpholium and an acetate anion wherein the physicalabsorption solvent is a dimethyl ether of polyethylene glycol.
 2. Thecomposition of claim 1 wherein said cation is a tetraalkyl phosphonium.3. The composition of claim 1 further comprising water.
 4. Thecomposition of claim 1 wherein said composition comprises from about1-45 vol % physical absorption solvent.
 5. The composition of claim 1wherein said composition comprises from about 10-35 vol % physicalabsorption solvent.
 6. The composition of claim 1 wherein saidcomposition comprises from about 10-30 vol % of said physical absorptionsolvent.
 7. The composition of claim 1 wherein said compositioncomprises from about 10-20 vol % of said physical absorption solvent.